How do these results on D and 3He relate to primordial
abundances?
Since D is mainly destroyed by astration, its proto-solar abundance
can be taken as a lower limit, but estimates of how much lower than
primordial are rather model-dependent with respect to Galactic
chemical evolution, depending on how much of the original Galaxy is
still in the form of gas (10 to 20 per cent in the Solar
neighbourhood), how much gas is returned to the ISM by each generation
of stars (10 to 40 per cent) and whether there has been significant
inflow of unprocessed gas from outside (cf.
Pagel 1982).
Thus factors
from 2 to 10 are all quite conceivable. In the case of 3He, the
situation is still more difficult because 3He is destroyed in
astration through massive stars, but survives and can also be freshly
produced in stars of lower mass
(Dearborn, Schramm &
Steigman 1986).
Yang et al. (1984)
pointed out that, because destruction of D
leads to production of 3He, some of which survives further
stellar
processing, one can use existing abundances in the proto-Solar System
to constrain the sum of primordial D and 3He by the equation

(9)

(10)

(11)

where y23p is the sum of primordial (D +
3He) / H and f is the fraction
of 3He that survives astration through one generation of
stars. In a
somewhat more sophisticated treatment the second limit is increased to
10.9 × 10-5
(Olive et al. 1990),
which is shown to a good approximation
by a horizontal line in fig. 1.
This gives what is currently the most stringent lower limit to
in the framework
of BBNS, shown by the
left-most tall vertical line and the leftmost shorter double vertical
line in fig. 1 for homogeneous and inhomogeneous models respectively.
Equation (9) refers to closed Galactic chemical evolution models
without inflow of unprocessed material. In models with inflow, which
have some distinct advantages
(Pagel 1989a),
different arguments apply
but the result is the same, since in such models it is virtually
impossible to have less than 1/3 of primordial deuterium surviving (cf.
Audouze & Tinsley 1974;
Pagel 1982).

It would be nice to have an independent upper limit to primordial
deuterium and there is a possibility that this will eventually be
achieved from observations of absorption-line systems at high
red-shifts having low metallicity, low Doppler broadening and large
hydrogen column density, in front of quasars
(Webb et al. 1991).
The absence of any very definite results on deuterium in such clouds up to
now suggests that the limit of 10-4 is not too likely to be
violated.